1. Field of the Invention
This invention relates to textiles designed for converting electrical charges, such as charges generated through solar energy, into usable electricity. More specifically the invention is directed to electrical charge transfer textiles, photovoltaic systems, solar textiles, and sub-components which reduce electrical resistance for improved performance.
2. Description of Related Art
Photovoltaic systems convert sunlight into electricity through the action of photovoltaic cells. Large solar arrays currently in use typically have numerous panels or modules, each with many photovoltaic cells. Such arrays have been made from rigid components. More recently, flexible photovoltaic components have been developed that may be incorporated into textiles as alternatives to rigid cells and modules.
Flexible solar energy technology such as polymer photovoltaics (PPV's) holds great promise for many applications. The freedom of movement provided by textiles has the potential for making solar energy conversion structures that are more easily transported and erected than comparable rigid solar structures. Such systems could be used to bring much needed electricity to remote or disaster ridden areas that would otherwise be without power. In other applications, efficient solar textiles integrated into common articles such as hats, garments, tents, and coverings could potentially provide electric power on a smaller scale.
Small cross-section photovoltaic fibers used for solar textile applications provide uniformity and fabric-like flexibility. Inexpensive and flexible polymer photovoltaics (PPVs) are well suited for use as fibers. However, attempts at producing efficient solar textiles from existing PPV components have been constrained by fundamental technical barriers relating to their inherent electrical resistance.
Present PPV fibers of coaxial construction rely on a centered inner conductor and a transparent external conductor, such as ITO (Indium Tin Oxide) or a conducting polymer such as PEDOT (Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)), to move charge along the fiber. However, when such fibers are incorporated into a textile, the low electrical conductivity of the external, optically transparent electrode causes significant voltage drop in the available electricity. The voltage drop results because the transparent electrode provides two critical but contradictory functions. The first function is to pass solar flux unimpeded through the transparent electrode into the optically active photoelectric layers beneath the surface. The second function is to move or transport electric charge axially along the sheet dimension of the transparent electrode with minimum voltage drop or loss. Efficient optical transmission requires maximum optical clarity, implying a relatively thin electrode. However, efficient charge transport requires sufficient thickness to provide a low electrical resistance path. While one function optimizes with increasing thickness the other optimizes with decreasing thickness. Currently available optically transparent compounds, such as ITO and many of the new polymer-based substances such as PEDOT, do not simultaneously satisfy the optical clarity and electrical conductivity requirements. For PPV components made from these substances, acceptable optical transmission results in excessive electrical sheet resistance for use in solar textiles.
Other photovoltaic fiber designs rely on dual internal conductors throughout their length. However, the movement of power through textiles made from such fibers is generally more complicated and less reliable because of the need to make and maintain additional electrical connections with external circuitry. Furthermore, the small cross sectional dimension typical of internal conductors restricts charge flow. Similar to co-axial fibers, charge transport along the axis of dual internal conductor fibers yields large voltage drop, thereby diminishing the performance of textiles in which such fibers are used.
Attempts at producing photovoltaic (PV) fibers for textiles have reported power conversion efficiencies of only 0.01% with electrical fill factors of 24%. (A Photovoltaic Fiber Design for Smart Textiles, Textile Research Journal Vol. 80 (11); 1065-1074 DOI. It has also been reported that: “An n-type carrier counter electrode that is both highly conductive and optically transparent has not been reported. Even indium-tin oxide coatings with a resistivity as low as 10 ohm/cm2 cannot transport the photocurrent generated with 1 sun irradiance over more than 10 to 15 mm without incurring electrical losses.” (Solar Power Wires Based on Organic Photovoltaic Materials, Science Magazine, 10 Apr. 2009.)
In addition to poor efficiency, solar textile modules made from photovoltaic fibers known in the art are subject to malfunction from shorting of conductors, particularly at connections where charges from multiple fibers are merged. Unless substantially fortified, the delicate nature of the small connections allows them to be damaged from minor impacts or abrasions. Depending on the design, a single short circuit could impair the function of multiple cells, or even adjacent solar modules. Similarly, solar textile modules made from other existing photovoltaic components such as thin films are either too fragile or rigid and are still largely unproven for exploiting the advantages of solar textiles.
A prior design developed and disclosed in PCT application serial number PCT/US2012/054866 provides a combination of components arranged to form a textile capable of generating electricity from solar energy. That prior design used a combination of highly conductive bus bars serving as conduits in contact with photovoltaic tapes or fibers to move charge in and out of textile unit cells. The bus bar conduits are arranged to minimize charge transport resistance throughout the textile by providing multiple, durable electrical contacts with charged surfaces along the length of the photovoltaic components. Unfortunately, the configurations of combinations of PV tapes and bus bar conduits woven together in an interlacing manner is inefficient in light of the present invention. Specifically, the PV tapes am overshadowed as a result of the over-and-under routing of the bus bar conduits as they pass over the top electrodes of the PV tapes. This limitation can result in shadowing of about 50% of the functional PV tape area. Textiles utilizing the prior design disclosed in the cited PCT application are also difficult to manufacture because they rely on multiple components, which increases the cost and complexity of fabrication and reduces the likelihood of widespread commercialization.
These and other technical problems relating to existing photovoltaic components and systems continue to inhibit the rapid commercialization of new applications for solar textiles. Therefore, there is a need in the art for more efficient photovoltaic textile components and materials in general particularly those that are durable but still flexible enough to exhibit the properties of fabrics.